The present invention relates to a sintering method and a sintering apparatus for use in the sintering method.
A sintering apparatus is known which is of a type sintering a powder material by applying a current to a mold while applying pressure thereto. The present inventor has previously developed a sintering apparatus of a current-applying type, as shown in FIGS. 19 and 20, which comprises a cylindrical mold (e.g., made of carbon or graphite; an outer diameter: about 180 mm; coaxial length: about 60 mm) 102 for accommodating a powder material 101, upper and lower punches 103a and 103b, respectively, disposed in the mold 102 so as to be movable for applying pressure to the powder material 101 filled in the mold 102, and a pair of electrodes 104a and 104b for forming the powder material 101 into a sintered article by applying a current to the mold 102 from the side thereof (as indicated by the broken-lined arrow in FIG. 20) and applying heat to the powder material 101.
As the sintering process apparatus of this current-applying type can lower a sintering temperature by elevating a pressure to be applied to the powder material 101 by means of the upper and lower solid punches 103a and 103b, respectively, the sintering process can reduce influences of the sintering temperature upon a rate of the oxidative loss of the mold 102 and the like as well as shorten the time for cooling the mold and the like after sintering, up to a level that can make the rate of the oxidative loss negligible. As a consequence, the sintering process of this type can control the oxidative loss in the mold 102 and the like and shorten a cycle time for sintering processes.
As a result of further extensive research on the sintering process conducted by the present inventor, it was found that the rise of the temperature at positions P1 and P2, which are close to contact locations where the electrodes contact with the side surfaces of the mold, is faster than the rise of the temperature at positions P3 and P4 apart from the contact locations between the electrodes and the side surfaces thereof, and consequently that a local temperature difference may be caused to occur to some extent between the positions, resulting in the fact that a portion where the temperature rise is the highest (around the position P1) is caused to reach the sintering temperature faster than the other portions thereof while maintaining the entire state as it is. This may cause a partially non-sintered portion to be formed in a sintered article as a product (as shown in FIGS. 20 and 21). It is further found that this tendency may become higher as the application of a current at the time of a rise is increased to a higher level in order to shorten the processing time. With this finding taken into account, the present inventor has come to recognition that improvements in this point are to be made in order to improve properties of a sintered article such as strength and the like.
The present invention has been completed on the basis of this finding and it has the object to form a sintered article so as to cause a local temperature difference to occur to the smallest possible extent at the time of sintering a powder material by applying a current thereto.
In order to achieve the object as described above, the present invention as claimed in claim 1 provides a sintering method comprising supplying heat to a powder material filled in a cylindrical mold for accommodating the powder material under pressurized condition by applying a current to the mold with whose surface a pair of electrodes are in contact, wherein:
positions of the pair of electrodes in which they are in contact with the side surfaces of the mold for applying the current are disposed so as to vary with time.
Preferred modes of this embodiment according to the present invention as claimed in claim 1 include modes of the embodiment according to the present invention as claimed in claims 2 to 7.
In order to achieve the object as described above, the present invention as claimed in claim 8 provides a sintering method for sintering the powder material by applying a current thereto, thereto wherein the application of the current is partially suspended.
Preferred modes of this embodiment of the present invention as claimed in claim 8 include modes of the embodiment according to the present invention as claimed in claims 9 to 16.
In order to achieve the object as described above, the present invention as claimed in claim 17 provides a sintering apparatus for sintering the powder material in a cylindrical mold for accommodating the powder material under pressurized condition, in which plural groups, each group composed of a pair of electrodes, are disposed in contact of the side surface of the mold and on the periphery of the mold in such a manner that a current is alternately applied to each group composed of a pair of electrodes.
In order to achieve the object as described above, the present invention as claimed in claim 18 provides a sintering apparatus for sintering the powder material around a cylindrical mold for accommodating the powder material under pressurized condition through plural groups, each group composed of a pair of electrodes disposed on the periphery of the mold, wherein each group composed of a pair of electrodes is allowed to alternately come into contact with the side surface of the mold and apply the current to the mold. Preferred modes of the embodiments of the present invention as claimed in claims 16 and 17 include modes of the embodiments of the present invention as claimed in claims 23 and 24, respectively.
In order to achieve the object as described above, the present invention as claimed in claim 18 provides a sintering apparatus, comprising:
a pair of the electrodes disposed around a cylindrical mold for accommodating the powder material under pressurized conditions and for applying heat to the powder material by applying a current to the side surface of the mold;
current-application adjustment means for adjusting the application of a current to the pair of the electrodes from a power source; and
control means for controlling the current-application adjustment means to maintain the pair of the electrodes in a current-applying state with respect to the power source in a usual case, in which the current is being applied to the electrodes from the power source, and partially bring the pair of the electrodes in a current-suspending state with respect to the power source, in which the application of the current from the power source is being suspended. Preferred modes of the embodiment of the present invention as claimed in claim 18 include modes of the embodiments of the present invention as claimed in claims 19 to 24.
The sintering method according to the present invention as claimed in claim 1 can positively apply heat (current) even to a portion of the mold where the temperature rises at a lower level because contact points between the pair of the electrodes and the side surface of the mold through which the current is applied are configured so as to vary with time by focusing on the fact that the temperature can rise to a higher level at the contact points between the electrodes and the side surfaces of the mold. This configuration of the sintering method can fail to cause a temperature difference to occur to the least possible extent in the mold during the sintering process and therefore provide a sintered article without causing the temperature difference to occur to the least possible extent during the sintering process.
The sintering method according to the present invention as claimed in claim 2 is configured such that three or more electrodes are disposed on the periphery of the mold in a spaced relationship apart from one another in a peripheral direction of the mold, and different two electrodes are optionally selected variably from such three or more electrodes, as an elapse of time, to form a pair of electrodes which are brought into contact with the side surface of the mold. This configuration of the sintering method can vary the contact points of the optionally selected pair of the electrodes with respect to the side surface of the mold with an elapse of time, so that, in this case, too, heat (current) can be positively supplied to a portion of the mold where the temperature rises at a lower level to allow the mold to cause a temperature difference partially to the lowest possible extent during the sintering process and, as a consequence, can form a sintered article while causing the temperature difference to fail to occur at the lowest possible level during the sintering process.
The present invention as claimed in claim 3 provides the sintering method which is configured such that a group consisting of a pair of electrodes is disposed on the periphery of the mold and the group of the pair of the electrodes is disposed so as to deviate its positional relationship relative to the mold in the peripheral direction of the mold as time elapses. This configuration can change the contact points of the pair of the electrodes with the side surfaces of the mold, at which the current is applied to the mold for sintering, as time elapses. Therefore, this sintering method can positively supply heat (current) to a portion of the mold where a rise in the temperature is slower, too thereby causing a local temperature difference to occur in the mold to the least possible extent at the time of sintering and producing a sintered article without causing a partial occurrence of the local temperature difference in the mold.
Further, this embodiment requires only two electrodes so that the number of electrodes can be minimized as small as possible for applying the current to the mold.
The present invention as claimed in claim 4 provides the sintering method wherein the three or more electrodes are all in contact with the side surfaces of the mold; and the two optional electrodes are selected therefrom by shifting the application of the current thereto. This configuration can achieve the same action and effects as achieved by the present invention as claimed in claim 2. Further, this embodiment of the present invention can cause no delay in rise because the disposition of the three or more electrodes does not require contact with or separation from the side surfaces of the mold for selection of the optional two electrodes. If the electrodes are otherwise separated apart from the mold, some period of time is required until heat is supplied to the mold after the electrodes are allowed to contact therewith and then the temperature of the mold is elevated because the temperature of the mold is caused to be lowered while the electrodes are separated from the mold. Therefore, this embodiment does not cause the temperature of the mold to vary (to be lowered) to a great extent upon shifting the electrodes, so that the temperature difference of the mold can be controlled to an appropriate level.
The invention as claimed in claim 5 further provides the sintering method wherein the two optional electrodes are selected by causing the electrodes to contact with or separate apart from the side surface of the mold. Therefore, this embodiment of the present invention can achieve the action and effects more specifically as achieved by the embodiment of claim 2.
The sintering method of the present invention as claimed in claim 6 is configured such that three or more electrodes are disposed; two groups of pairs of electrodes are selected from the three or more electrodes; and said two groups of the pairs of the electrodes are disposed so as for a virtual line connecting a one group consisting of the pair of the electrodes to each other to intersect another virtual line connecting the other group consisting of the pair of the electrodes of another group to each other at the substantially right angle; wherein the current is alternately applied to the groups of the pairs of the electrodes. This configuration can readily control each of the groups of the pairs of the electrodes by predetermining each of the pairs of the electrodes as each group. In addition, this configuration can effectively control an occurrence of a local temperature difference in the mold at the time of sintering by minimizing the number of the electrodes as small as possible.
The present invention as claimed in claim 7 provides the sintering method wherein the current is applied to the one group of the pair of the electrodes selected from the two groups thereof at an initial time of applying the current until the temperature of the mold reaches a predetermined temperature; and thereafter the current is intermittently applied at small time intervals alternately to each of the two groups of the pairs of the electrodes. Therefore, the temperature difference in the mold can be made as small as possible and the processing time can be shortened to some extent, so that the above two features can be satisfied greatly.
Further, the present invention as claimed in claim 8 provides the sintering method for sintering the powder material filled in the mold by applying a current to the mold in a manner that the current applied to the mold is partially suspended. This configuration can transfer heat (perform heat conduction) from the high-temperature portion of the mold to the low-temperature portion thereof to reduce local temperature difference even if a higher portion of raising the temperature in the mold would be caused to occur locally by sintering upon the application of the current thereto. Therefore, a sintered product can be produced without causing local temperature difference to the highest possible extent at the time of sintering, while utilizing heat effectively.
The sintering method as claimed in claim 9 is provided with the step of supplying heat to the powder material filled in the cylindrical mold under pressurized conditions for sintering by applying the current to the mold while the pair of the electrodes are in contact with the side surfaces of the mold. Therefore, this configuration presents the situation that a portion where the temperature becomes higher than the other portions is likely to be located due to a higher rate of elevating the temperature locally on account of the side surface of the mold where there is the limitation on processing precision or the like, a small contact area between the side surface of the mold and the electrodes, etc.; however, the suspension of the application of the current permits the heat to transmit from the higher-temperature portion to the lower-temperature portion in the mold, and a sintered article can be produced in such a manner that local temperature difference is reduced to the least possible extent at the time of sintering.
Moreover, the present invention as claimed in claim 9 provides the sintering method wherein the pair of the electrodes are separated apart from the side surface of the mold upon suspending the application of the current for sintering the powder material. This configuration can prevent heat in the mold from leaking toward outside through the electrodes at the time when the application of the current is suspended. Therefore, the heat present in the mold can be effectively utilized upon transferring the heat in the mold from its higher-temperature portion to its lower-temperature portion.
Additionally, the sintering method as claimed in claim 10 is configured such that, as each of the electrodes is disposed so as for a tip portion thereof to come closer to or separate apart from the main body of the mold in such a manner that, when the tip portion thereof is separated apart from the main body thereof, a space area is formed between the tip portion thereof and the main body, heat in the mold can be prevented from escaping through the electrodes by the location of the heat-insulating space area, even if the electrodes are kept in contact with the side surface of the mold. Therefore, the heat present in the mold can be utilized to a highly effective extent upon transferring the heat from the higher-temperature portion to the lower-temperature portion thereof.
The present invention as claimed in claim 11 provides the sintering method which is configured in that three or more electrodes are disposed in a peripherally spaced relationship on the periphery of the mold, that a pair of two optional electrodes are selected from the three or more electrodes by shifting the electrode, as time elapses, and that, on selecting the pair of two optional electrodes by shifting said electrodes, a period of time during which the application of the current is partially suspended is provided for suspending the application of the current for sintering the powder material. Therefore, the timing of shifting the electrodes can be effectively utilized for correcting the temperature in the mold and making the temperature uniform in the mold, and the shifting of the electrodes can be effected in a smooth way.
In a preferred mode of this embodiment, the sintering method as claimed in claim 13 is configured such that the mold is made of graphite. This configuration can provide the mold with thermal resistance, thermal shock resistance, and conductivity at such a level as required for use as a mold. This configuration, however, may otherwise create the situation in which a portion is caused to locate in the mold where the temperature is higher than the other due to the fact the rate of transferring heat in the mold is slower as compared with the rate of supplying heat from the electrodes. Therefore, in this situation, the suspension of the application of the current to the mold can accelerate the heat conduction from the higher-temperature portion to the lower-temperature portion to reduce local temperature difference in the mold, and a sintered product can be produced in a situation where such local temperature difference is very low at the time of sintering.
In addition, in a preferred mode of the embodiment, the present invention as claimed in claim 13 provides the sintering method, wherein the application of the current is suspended when local temperature difference between two predetermined positions of the mold reaches a predetermined temperature difference or larger. Therefore, this mode of the embodiment according to the present invention can correct the temperature in the mold so as to make the temperature in the mold as uniform as possible, while regulating the local temperature difference in the mold from becoming too large by supplying heat for sintering the powder material by means of the application of the current thereto.
Furthermore, in another preferred mode of this embodiment, the present invention as claimed in claim 14 provides the sintering method, wherein pressure is applied to the powder material in a state in which the application of heat from the outside is insulated, by taking advantage of the feature that no application of the current is to be performed on the side of applying pressure thereto. This configuration can prevent heat present in the mold from escaping through pressurizing means such as pressurizing punches or the like, so that the heat in the mold and so on can be utilized effectively upon transferring heat in the mold from the higher-temperature portion to the lower-temperature portion thereof.
Still further, in another preferred mode of this embodiment, the present invention as claimed in claim 15 provides the sintering method, wherein the application of the current to the mold is suspended plural times. This configuration can perform the suspension of the application of the current to the mold more effectively for correcting the temperature of the mold toward making the temperature in the mold uniform.
In another aspect, the present invention as claimed in claim 16 provides a sintering apparatus for sintering a powder material filled in a cylindrical mold by alternately applying a current to the mold through a pair of electrodes disposed in contact with the peripheral side surfaces of the mold, wherein plural groups, each group composed of a pair of electrodes, are disposed on the periphery of the mold while being in contact with the side surface of the mold; and each group composed of the pair of electrodes are selected to alternately apply the current to the mold. Therefore, the sintering apparatus in this embodiment can be configured so as to change the contact points at which the pair of the electrodes come into contact with the side surface of the mold in accordance with a lapse of time so that the present invention can specifically provide the sintering apparatus that can practice the sintering methods according to the embodiments as claimed in claims 1, 2, 4, 6 and 7.
Further, the sintering apparatus for sintering the powder material filled in the cylindrical mold as claimed in claim 13 is likewise configured such that the mold is supplied with the current through a pair of electrodes disposed in contact with the peripheral side surface of the mold, wherein plural groups, each group composed of a pair of electrodes, are disposed on the periphery of the mold and each group is selected so as to come into contact with the side surface of the mold and apply the current to the mold. In this embodiment, too, the sintering apparatus in this embodiment can be configured such that the contact points at which the pair of the electrodes come into contact with the side surfaces of the mold can be varied with a lapse of time, thereby providing the apparatus that can specifically practice the sintering methods according to the embodiments as claimed in claims 1, 2, and 5.
Moreover, the sintering apparatus of the present invention as claimed in claim 18 is provided with a pair of electrodes disposed on the periphery of the cylindrical mold for accommodating the powder material under pressurized conditions for applying heat to the powder material by applying the current to the mold; a current-application adjusting means for adjusting application of the current to the pair of electrodes from a power source; and a control means for controlling the current-application adjusting means to assume a current-applying state at a usual time in which the current is supplied to the pair of electrodes from the power source and a current-suspending state in which the application of the current to the pair of electrodes is partially suspended. Therefore, the sintering apparatus according to this embodiment can partially suspend the application of the current to the mold so that the it can specifically practice the sintering method as claimed in claim 8.
Furthermore, the present invention as claimed in claim 19 provides the sintering apparatus wherein pressure can be applied to the powder material from both sides of the mold in axial directions by means of a pressurizing punch having a heat insulating layer by utilizing the feature that the current is not required to be applied on the side of the pressurizing punch. Therefore, this configuration can control the heat in the mold from escaping through the pressurizing means so that the heat in the mold and so on can be utilized effectively upon transferring heat from the higher-temperature portion of the mold to the lower-temperature portion thereof. This configuration can provide the sintering apparatus that can specifically perform the sintering method as claimed in claim 14.
In addition, in a preferred mode of the above embodiment, the sintering apparatus as claimed in claim 10 is configured such that the mold is made of graphite. This configuration can provide the mold with thermal resistance, thermal shock resistance, and conductivity at such a level as required for use as a mold. This configuration, however, may otherwise create the situation in which a portion is caused to locally locate in the mold where the temperature is higher than the other due to the fact the rate of transferring heat in the mold is slower as compared with the rate of supplying heat from the electrodes. In this situation, the suspension of the application of the current to the mold can accelerate the heat conduction from the higher-temperature portion of the mold to the lower-temperature portion thereof to reduce a local temperature difference in the mold. Accordingly, this embodiment can provide the sintering apparatus that can specifically perform the sintering method as claimed in claim 12.
In a preferred mode, the sintering apparatus of the present invention as claimed in claim 22 is configured such that a group of the pair of the electrodes are selected from plural groups of pairs of electrodes which are disposed so as to be shifted in sequence and the control means is set so as to control the current-application adjusting means to implement the current-application suspending state, when it is judged to perform the shift of the electrodes. This arrangement of the sintering apparatus can utilize the timing of performing the shift of the electrodes for correction of the temperature of the mold for making the temperature in the mold uniform and perform the shifting of the electrodes in a smooth way. Therefore, this embodiment of the present invention can provide the sintering apparatus that can specifically practice the sintering method as claimed in claim 11.
Further, the present invention as claimed in claim 12 provides the sintering apparatus further comprising a mold temperature detecting means for detecting the temperature of the mold in plural positions; wherein the control means is set so as to perform the current-suspending state by controlling the current-application adjusting means when it is judged that a temperature difference in two positions out of the plural positions reaches a predetermined temperature difference or larger on the basis of a signal from the mold temperature detecting means. This configuration of the sintering apparatus can regulate the temperature difference from becoming too large by the application of heat on the basis of sintering by the application of the current to the mold. Moreover, it can correct the temperature of the mold for making the temperature in the mold uniform.
Therefore, this embodiment of the present invention can provide the sintering apparatus that can specifically perform the sintering method as claimed in claim 13.
In a preferred mode of the above embodiments of the present invention, the sintering apparatus as claimed in claim 23 is configured such that a temperature detector is disposed so as to contact with or separate apart from the side surface of the mold. This configuration can detect the temperature of the mold in an accurate way simply by allowing the temperature detector to be in contact with the side surface of the mold, and serve as automating the detection of the temperature in the mold. At the same time, this configuration serves as saving mounting work for mounting the temperature detector such as a thermocouple or the like on the mold, and, if the temperature detector would be mounted thereon, it can prevent an error in measurement from becoming large.
In another preferred mode of the above embodiments, the present invention as claimed in claim 14 provides the sintering apparatus, wherein a thermocouple is disposed in a tip portion of the electrode. The electrodes can serve as a temperature detector so that the temperature of the mold can be measured by allowing the electrodes to contact with the side surfaces of the mold. Therefore, this embodiment of the sintering apparatus can achieve the action and effects as that as claimed in claim 23 and at the same time serve as simplifying the apparatus.